Indirectly heated cathode



Dec. 19, 1933. s. LOEWE 1,939,997

INDIRECTLY HEATED GATHODE Filed June 25, 1928 7 ELi KW m Patented Dec. 19, 1933 UNITED STATES PATENT OFFICE Application June 23, 1928, Serial No. 287,812,

r and in Germany June 25, 1927 2 Claims.

It is a well known fact. to indirectly heat glow ing cathodes of vacuum tubes. An example is afforded by the well known arrangement in which an electric field is arranged between a glowing auxiliary cathode and the main cathode when the main cathode is heated by electron bombardment. Other well known methods of indirect heating base on heating a main cathode which, for instance, is of the shape of a cylinder jacket, by heat radiation by means of a glower arranged inside. In the last mentioned instance it'is also known to place the, for instance cylindrically shaped, main cathode around the heaters arranged inside the cylinder so that the heat passes to the cathode also by heat conduction. In all known cases, however, it is the main cathode itself which is heated by the source of heat.

My invention now consists in the main cathode not being itself heated by the source of heat, but in heating it by the source of heat of a third element which is in good heat-conducting connec-- tion with the main cathode to be heated. In this manner the advantage is obtained that the main cathode may be, for instance, a solid wire which offers maniifold advantages inthe construction of tubes.

My invention will be best understood with reference to the accompanying drawing, in which Fig. 1 shows by means of example an arrangement by which the principle of my invention may be understood, whereas Fig. 2 illustrates an arrangement in which small auxiliary metallic bodies, connected to the cathode, may be heated by electron bombardment. Fig. 3 shows a somewhat modified form of such an arrangement, in which such metallic bodies are formed by a screw-shaped wire, and Fig. 4 shows an arrangement in which the heating of the cathode is made by means of connections in which electrical currents are running.

In Figs. 1, 2, 3, and 4 systems of three electrodes,

a plate, a grid, and a cathode areshown together with means for heating said cathode. Fig. 5 shows a complete three-electrode-valve in which a cathode containing such three-electrode-systems with a cathode according to my invention is arranged.

In Fig. 1 l is the main cathode, e. g. a solid tungsten wire. By thespiral 2 a control grid is indicated, by 3 the anode. 1, 2 and 3 represent together an amplifying system of the customary kind with a grid 2. In such a system means are provided for making cathode l glowing. According to my invention this cathode may be heated by heat conduction in such manner that it is in good vmay likewise be made to glow by electronnantly be heated from one side so that one of its ends would be considerably warmer than the 'manner effected in part by heat radiation on the heat-conducting connection with a body 4 which in its turn is made to glow by any method whatsoever. Body 4 may therefore be heated in any chosen manner. The heatingmay be effected by J oule-heat in which case the body 4 is made as a current carrying heating spiral. All other methods of heat generation may be used too. Body 4 may, for instance, be a metal cylinder which is made to glow by means of a coil slipped over and carrying low or high frequency currents. Body 4 bombardment. In the latter case it would appropriately be given the form of a hollow cylinder with which the main cathode 1 is heat-conductingly connected. Inside the hollow. cylinder the glowing cathode is to be arranged and a direct voltage or alternating voltage is to be connected between the latter and the tinplate cylinder 4. 7

It is obvious that with the arrangement according to Fig. l the main cathode would predomiother. This may easily be changed by the heating being efiected from both sides.

Fig. 2 illustrates such arrangement with the use of the electron bombardment for the indirect heating of bodies 4. Main cathode 1 is at both ends heat-conductively connected with cylinders 4 and, as in Fig. 1 surrounded by grid 2 and the cylindricalanode 3. Axially through cylinders two incandescent wires, indicated by dots and consisting, for instance, of tungsten wire, are placed which are made to glow by direct or alternating current. Between the cylinders 4 connected with one another by the lead 5 and the incandescent wires inside of them a direct or alternating voltage is connected. (Actually a special connection 5 is not required since the cylinders are already conductively connected by cathode 1). Thereupon a heating of the cylinders 4 is in well known part of the axial incadescent wires and in part by electron bombardment. The cylinders should consist of tantalum, molybdenum or tungsten sheet in order that they may take very high temperatures. The main cathode 1 is then from both ends made to glow by heat coduction so that the amplifying system 1, 2, and 3 operates like a normal incandescent filament amplifying system. The main cathode 1 more particularly may also consist of a thor'iated tungsten wire.

The arrangement according to Fig. 2 affords the possibility of providing screens 6 which are, for instance, circular and centrally perforated mica discs which prevent the penetration of stray 119 electrons from the inside of cylinder 4 into the inside of amplifying system 1, 2, 3. The employment of such screens is, however, recommended also for all other embodiments of the invention and is always possible.

A practical embodiment of the arrangement according to Fig. 2 is illustrated in Fig. 3. Here cylinders 4 are produced by cathode 1 being at both ends wound up to a cylindrical spiral 7. These spirals, of course, are made to glow very brightly, e. g. to 2300 by the electron bombardment. By heat conduction part 1 extending through amplifying system 2, Bis heated from both ends and, as tests have proved, may easily be heated sufilciently to the temperature of about 1700 necessary for the formation of the thorium oxide (oxide cathode), considerably lower temper-- atures will, of course, suflice. Actually spirals 7 are'woun'd very closely and with closely approaching turns. The employment of the electron bombardment according to Figs. 2 and 3 has a special advantage. As with amplifying tubes operated with supply connection to the alternating current mains generally simultaneously a rectification is required for purposes of plate voltage, special rectifiers may be dispensed with if this heating method isapplied. On the contrary alternating current rectified in connection with the electron bombardment of cylinders 4 or spirals 7 may be employed for further use, If the well known filter circuits and chains are interposed, then by using the direct current thus produced both the plate voltage required for operating the-amplifying system 1, 2, 3, and the filament voltage rectified for operating final loudspeaker stages may be obtained. For heating the final stages of multi stageamplifying. unitsthe rectified currents obtained as explained above, it is true, suiiices completely'as the amplification is no longer very high in these final stages. In the case of a three stage, resistance coupled amplification arrangement it is, for instance, only necessary to indirectly heat the first and second system by the method explained in Figs. 2 or 3 whilst the finalstage (loudspeaker tube) may be fed directly with the rectified current obtained in the systems 7 and to some extent steadied by means of filter circuits. It is, of

course, possible to take all rectified plate voltages from this source.

Although the arrangements according to Figs. 2 and 3 with the use of the electron bombardment have certain advantages, this, however, is also true with the further embodiment of the invention illustrated by Fig. 4. Here the main cathode 1 is simply extended on both sides, the current supply being, however, arranged in such manner that only its two, outer ends 8 and 9 are traversed by heating current and are therefore made to glow brightly. The heating current flows in at 10 and out at ll and 12. In this case, of course, the current is alternating current. Parts 8 and 9 are thus made to glow brightly. The inner part 1 of the cathode is heated by heat conduction from both ends. This part, of course, is at a considerably lower temperature which, however, is still high enough to provoke the thorium or oxide emission.

The advantage of this arrangement, however, consists in the fact that along cathode 1 no drop of potential takes place. Provided the drop of volt-- age in the parallel leads 13, 1a is equally high, this partlies on points of like potential. For that reason, even if the glowing current itself is al ternating current, no drop of potential takes place along main cathode 1 in consequence of the network heating so that no alternating current noises become noticeable in the amplifying systems 1, 2, 3. The main dii'liculty in the operation of sensitive amplifying arrangements with supply heating is therefore overcome also in the case of adapted to heatsaid metallic bodies by electron bombardment.

2. An electron discharge device containing a thermionic cathode, one tinplate cylinder arranged at each end of said thermionic cathode, a heating wire arranged inside of each tinplate cylinder, said heating wires being connected with one another, said heating wires being adapted to heat said cylinders by electron bombardment.

. SIEGMUND LOEWE. 

